It is well-known that the process of nuclear fission involves the disintegration of the fissionable fuel material, usually enriched uranium dioxide, into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as "burnable absorbers" if they have a high probability (or cross-secton) of absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation, the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross-section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.
The incorporation of burnable absorber in fuel assemblies has been recognized in the nuclear industry as an effective means of increasing fuel capacity and thereby extending core life. Burnable absorbers are used either uniformly mixed with the fuel (i.e., distributed absorber) or are placed discretely as separate elements in the reactor, as separate burnable absorber rods, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core.
Among the various burnable absorbers that have been mixed with fuel as a distributed absorber, gadolinium oxide has been found to be an excellent absorber due to its extremely high thermal absorption cross-section. Enriched uranium dioxide, with a high U-235 isotope content, and gadolinium oxide, as a mixture, has thus been previously used in formation of nuclear fuel pellets.
Where gadolinium oxide is used as the burnable absorber in fuel pellets, a disadvantage exists in that the absorbing quality of the gadolinium oxide is relatively stable over a period of time but then decreases very quickly and is lost. In effect, the burn-out rate of gadolinium oxide is faster than desired. Also, a problem exists where higher amounts of gadolinium oxide are added to uranium dioxide in the manufacture of fuel pellets because of physical limitations such as clumping of the material during fabrication. The use of sintered microspheres of gadolinium oxide in a nuclear fuel has been suggested in U.S. Pat. No. 3,759,786, which discusses the use of coated gadolinium oxide bodies, such as molybdenum coated gadolinium oxide, prepared in microsphere form, as consumable absorbers in uranium dioxide. Uranium borides and borides of metals having a low neutron-capturing section, such as zirconium, prepared in microsphere form are also suggested for incorporation into uranium dioxide.
Boron compounds, such as boron carbide are also known for use as burnable absorbers. While boron compounds containing the isotope B.sup.10 are usable as burnable absorbers, they do not have the absorption of gadolinium oxide and thus must be provided in larger amounts, which thus must displace some fuel in pellets. Also, boron when used as a burnable absorber produces helium gas during burn-out, which gas produces undesirable pressures within the fuel element. In instances where boron compounds have been used in separate burnable poison rods, in conjunction with conventional fuel rods, coated boron carbide has been used, with a coating over the boron carbide particles to contain such helium. U.S. Pat. No. 3,356,618, for example, teaches the formation of coated boron particles where the boron particles have an inert coating, such as a carbide, nitride or carbo-nitride of a refractory metal such as zironium, formed thereon and the coated particles are dispersed in a metal matrix, such as iron, cobalt, nickel, aluminum and zirconium, for use in neutron absorption control elements for a nuclear reactor. In U.S. Pat. No. 3,855,061, which relates to a nuclear fuel plate containing microspheroidal particles of a fuel, the fuel particles may be coated with niobium, nickel, alumina, pyrolytic graphite or other materials. This patent also suggests that the microsphere route can be used to introduce nuclear poisons such as gadolinium or samarium to a fuel, or introduction of microspheres of a neutron poison, or neutron moderating materials such as boron carbide to the fuel when desired.
It is an object of the present invention to provide a nuclear fuel element which combines the benefits of gadolinium oxide and boron carbide as absorbers in a single fuel pellet.
It is another object of the present invention to provide a nuclear fuel which exhibits improved nuclear reactor power distribution control than is provided by the use of gadolinium oxide alone as a burnable absorber.